Antigen recognition by T lymphocytes is a complex process involving processing of the antigen into fragments by another cell and presentation of these to T cells in association with a major histocompatibility (MHC) molecule on the surface of that other cell. Therefore, only a few segments of a protein antigen will be seen by T cells, in contrast to antibodies. For vaccine development, it will therefore be important to locate such immunodominant antigenic sites. Fortunately, T cells have one advantage over antibodies in that T cells crossreact with short synthetic peptides of the protein much more effectively than do most antibodies. We have used synthetic peptides to characterize in detail the antigenic sites of a model protein antigen, myoglobin, recognized by murine T cells and T-cell clones established in this lab. These studies indicated that such sites tended to be amphipathic helices, i.e., helices that have hydrophilic and hydrophobic residues separated on opposite sides. We have developed a computer algorithm which locates potential amphipathic helical segments of proteins and requires knowledge only of the amino acid sequence. This algorithm identifies 18 or 23 known immunodominant T cell sites on 12 proteins (p less than 0.001). We applied this to two proteins of importance for vaccine design, the circumsporozoite protein (CSP) of malaria, and the HIV (AIDS virus) envelope. In both of these we predicted major T- cell antigenic sites, synthesized the corresponding peptides, and showed that these elicited T-cell immunity in mice. In the malaria case, we coupled the T-cell site to a known target site of neutralizing antibodies to construct a totally synthetic immunogen capable of eliciting antibodies in strains of mice that could not respond to the antibody site alone. These approaches should be useful in the rational design and construction of synthetic and recombinant fragment vaccines. Also, we used avidin blocking of presentation of biotinylated peptide to specific T-cell clones to demonstrate the presence of peptide on the cell surface where it was suspected but had not previously been demonstrate in most cases with antibodies to the antigen. Finally, we demonstrated that B lymphocytes very efficiently take up, process, and present antigens to T cells via their surface immunoglobulin, but that this immunoglobulin sterically hinders processing of part of the antigen and so leads to selective processing and presentation to only a subset of antigen-specific T cells. This may provide a mechanism to explain how immune response genes, which control T-cell specificity, can also indirectly control the specificity of antibodies produced, by selective T help of only those B cells producing the right peptides.